1. Field of the Invention
The present invention relates to a directional coupler using a non-radiative dielectric (NRD) waveguide, and more particularly, to a millimeter wave band non-radiative dielectric waveguide directional coupler using a multiple-hole structure in which two parallel NRD waveguides is located between upper and lower conductive plates and a conductive plate having a multiple-hole structure is inserted between two NRD waveguides to each other to couple electric and magnetic waves of an electric field component or a magnetic field component. This is mainly used in a RF (radio frequency) circuit of the millimeter wave band wireless communication system.
2. Description of the Prior Art
In the conventional directional coupler using the NRD waveguide, a method of determining the coupling ratio by adjusting the distance between two NRD waveguides is often used. As such method, there are three methods. A first method is a method of adjusting the coupling ratio by positioning two NRD waveguides having straight line shape and parallel to each other between the upper and lower conductive plates to adjust the distance between two NRD waveguides. A second method is a method of adjusting the coupling ratio by positioning two NRD waveguides bended with a predetermined curvature radius between the upper and lower conductive plates to adjust the distance between the two NRD waveguides. A third method is a method of adjusting the coupling ratio by positioning a NRD waveguide having a straight line shape and a NRD waveguide bended with a predetermined curvature radius between the upper and lower conductive plates to adjust the distance between the two NRD waveguides.
The conventional the directional coupler using the NRD waveguide mostly uses a coupling method for adjusting the distance between the two NRD waveguides. The directional coupler using this coupling method has a merit that the structure thereof is simple various coupling ratios can be implemented. However, it is difficult to fix the location for maintaining the adjusted distance, and it is difficult to reproduce it because the coupling distance is adjusted so as to have a desired coupled amount for each manufacture thereof, thereby it is difficult to adjust an accurate coupling ratio, and the curvature loss is generated in case of the directional coupler using the bended NRD waveguide.
On the other hand, beside the conventional directional coupler for determining the coupling ratio by adjusting the distance between two NRD waveguides, there is a branch-line coupler using the NRD waveguide. This is 3 dB directional coupler designed such that a series arm having an of (½)1/2 impedance of a characteristic impedance and a ¼ length of a guided wavelength (λg) and a parallel arm having an impedance equal to the characteristic impedance and a ¼ length of λg are arranged to output 3 dB of the half size of the input signal to a through port and a coupled port and the phase shift between two ports becomes 90 degree.
In case of the branch-line coupler using the NRD waveguide, there are advantages that it is easy to fix the location and the reproduction thereof is good. But, it is difficult to implement various coupling ratios, because only the 3 dB coupler can be implemented according to the characteristics of the coupler.